1. Field of the Invention
The present invention relates to a lens system, and particularly relates to a compact lens system for use in a compact photographic apparatus such as a camera phone and a compact digital camera.
2. Description of Prior Art
In the optical art, spherical lenses are widely used to facilitate manufacture and increase yield. However, spherical lenses have the inherent problems of optical aberrations such as spherical aberration and chromatic aberration. Accordingly, conventional lens systems are generally composed of several spherical lens elements to balance these inherent optical aberrations. However, where all lens elements have spherical surfaces, a high number of lens elements is generally required, thereby making the lens system long, heavy, and expensive to manufacture. This deviates from the current trend for compact photographic apparatus requiring a corresponding compact lens system having a very short overall length from the frontmost lens surface to the image plane.
Accordingly, aspheric lenses are introduced to help reduce aberrations. The inclusion of aspheric lens elements in a lens system enables reduction of the amount of lens elements in the system, which makes the lens system smaller and lighter.
Spherical or aspheric lenses may be made of plastic or glass. In comparison to plastic, glass has higher transmissivity, better performance, but higher cost, and thus is suitable for high-end applications. In contrast, plastic has lower transmissivity, lighter weight and lower cost, and thus is suitable for low-end applications. In some applications, a glass and plastic hybrid lens is employed for reducing the overall weight while maintaining relatively high performance.
In the current market, most lens systems for camera phones are generally composed of two plastic lens elements, or of one glass lens element and two plastic lens elements. The lens system of two plastic lens elements has low optical performance and requires a high level of precision, which limits its extensive application. The lens system of one glass lens element and two plastic lens elements is disclosed in U.S. Pat. No. 6,441,971. As shown in FIG. 1, the lens system 90 disclosed in the '971 patent includes, from the object side thereof, an aperture stop 91, a distal meniscus lens element 92 made of glass, first and second aberration correcting lens elements 93, 94 made of plastic, a cover glass 95 and an image plane 96. The meniscus lens element 92 has a convex surface facing the object side, and is adapted to reduce lens temperature sensitivity and provide most of the focusing power while the lens elements 93, 94 provide aberration compensation. The lens elements 93, 94 are both aspheric to reduce the overall length of the lens system 90. However, the aspheric lens elements 93, 94 are asymmetrically arranged. As a result, distortions of the optical system 90 cannot be eliminated completely. Furthermore, since most of the focusing power of the optical system is provided by the meniscus lens element 92, the meniscus lens element 92 must be formed with a large curvature, which makes manufacture difficult especially when the aperture of the meniscus lens element 92 is small.
U.S. Pat. No. 4,212,517 discloses a lens system of a modified Gauss type having the aperture stop between lens elements. This symmetry design of the lens elements allows well-balanced correction of all types of aberrations. However, this lens system is composed of six lenses. Consequently, the overall length of this lens system is increased, which deviates from the current trend for compactness. Various symmetry arrangements of lens elements are also disclosed in U.S. Pat. Nos. 4,364,643; 4,390,252; 4,396,255; 4,426,137; 4,443,070 and 4,44,8497. However, these lens systems as disclosed also have the same problem of a large number of lens elements, and thus are not suitable for use in compact photographic devices.